Created attachment 332[details]
debug
Now I think that I have a better log!
The resolver stay in loop. I've canceled in line:
[1467418844] libunbound[13863:0] debug: close fd 12
I started again and after few other tries the resolver got success.
log attached

Hi,
The domain keeps giving timeouts. It seems intermittently offline and not responding (to any IP address, lots of queries in sequence).
You have ipv6 enabled, and unbound starts exploring ip6 connectivity to the domain, which it says it has, but you haven't, and this wastes time. do-ip6: no could solve this in unbound.conf. Or you could enable IPv6, perhaps their IPv6 connectivity is better. If you do do-ip6: no in unbound.conf unbound can spend more time sending probes to the affected domain. Or enable IPv6 for the same effect and also unbound has more choice in servers that hopefully work.
The domain gives timeouts and has a short (720 second, a couple minutes) TTL. This causes the unstability you mention. Enabling prefetch may somewhat alleviate this, if the domain is prefetched such short periods of disconnection may become unnoticed, as unbound simply prefetches it again a couple seconds later.
The domain owner is urged, by the way, in several BCP statements to have the servers in independent locations, i.e. that don't go offline at the same time.
Best regards, Wouter